A plasmid DNA transformation system for a Lancefield Group F Streptococcus has been developed and used to assay for the presence of a restriction-modification system in this strain. These studies have also established that optimum competence occurs during the lag phase of the growth cycle. Examination of the microbial flora in human plaque samples has shown that treatment of patients with tetracycline results in a 100-fold increase in the percentage of cells resistant to high levels of the antibiotic. Such treatment also results in a 10- to 75-fold increase in the proportion of cadmium resistant isolates. With only a few exceptions, all tetracycline and cadmium resistant isolates have been streptococci. Preliminary studies have shown that these strains contain plasmids and efforts are currently being directed toward correlating the presence of plasmid with the resistance phenotype(s).